Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently colored light-emitting elements commonly referred to as sub-pixels, typically emitting red, green, and blue light, to represent each image element or pixel. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode (LED) displays.
Light emitting diodes (LEDs) incorporating thin films of light-emitting materials forming light-emitting elements have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 issued May 7, 2002 to Tang et al. shows an organic LED (OLED) color display that includes an array of organic LED light-emitting elements. Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, transport, or blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a pixel when current passes through the light-emitting material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
LED devices can include a patterned light-emissive layer wherein different materials are employed in the pattern to emit different colors of light when current passes through the materials. Alternatively, one can employ a single emissive layer, for example, a white-light emitter, together with color filters for forming a full-color display, as is taught in U.S. Pat. No. 6,987,355 entitled, “Stacked OLED Display having Improved Efficiency” by Cok. It is also known to employ a white sub-pixel that does not include a color filter, for example, as taught in U.S. Pat. No. 6,919,681 entitled, “Color OLED Display with Improved Power Efficiency” by Cok et al. A design has been taught employing an unpatterned white emitter together with a four-color pixel comprising red, green, and blue color filters and sub-pixels and an unfiltered white sub-pixel to improve the efficiency of the device (see, e.g. U.S. Pat. No. 7,230,594 issued Jun. 12, 2007 to Miller, et al).
Pixels can employ different arrangements of sub-pixels. For example, one arrangement locates the sub-pixels of a pixel in a row, forming colored stripes in the column direction. In some designs, neighboring rows are offset forming a delta pattern. In designs employing four sub-pixels in a pixel, the four sub-pixels can be arranged in a two-by-two array, also known as a quad pattern. Typically, the sub-pixels are equally spaced and are uniformly distributed over the substrate to form a regular array of equally spaced pixels.
Two different methods for controlling the pixels in a flat-panel display device are generally known: active-matrix control and passive-matrix control. In a passive-matrix device, the substrate does not include any active electronic elements (e.g. transistors). An array of row electrodes and an orthogonal array of column electrodes in a separate layer are formed over the substrate; the overlapping intersections between the row and column electrodes form the electrodes of a light-emitting diode. External driver chips then sequentially supply current to each row (or column) while the orthogonal column (or row) is supplied a suitable voltage to drive current through each light-emitting diode in the row (or column). Each sub-pixel is treated as a separately controlled element.
In an active-matrix device, active control elements are formed of thin films of semiconductor material, for example amorphous or poly-crystalline silicon, coated over the flat-panel substrate. The semiconductor materials are formed into thin-film transistors and capacitors through photolithographic processes. Thin-film transistors (TFTs) made from amorphous or polycrystalline silicon are relatively large and have lower performance compared to conventional transistors made in crystalline silicon wafers. Moreover, such thin-film devices typically exhibit local or large-area non-uniformity across the glass substrate that results in non-uniformity in the electrical performance and visual appearance of displays employing such materials.
Typically, each sub-pixel is controlled by one control element and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each light-emitting element typically employs an independent control electrode and an electrode electrically connected in common. Control of the light-emitting elements is typically provided through a data signal line, a select signal line, a power connection and a ground connection. Designers typically ensure that the active-matrix circuits and sub-pixel elements are uniformly distributed over the substrate. Each sub-pixel is treated as a separately controlled element.
Employing an alternative control technique, Matsumura et al., in U.S. Patent Application 2006/0055864, describe crystalline silicon substrates separate from a display substrate used for driving LCD displays. The application describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown. Compared to active-matrix circuits, chiplets are relatively large, although there can be relatively fewer of the chiplets than of the alternative thin-film active-matrix circuits. The larger chiplets can detrimentally affect the image quality of the display. Optimized arrangements of sub-pixels and pixels are not disclosed.
There is a need for an improved sub-pixel arrangement in displays employing driving circuits having silicon substrates separate from the display substrate.